1. Field of the Invention
The present invention relates to a method of detecting a focus defect such as a defocus value or astigmatic error of an electron microscope image for the purpose of correcting the focus defect,and more particularly to a method of detecting such a focus defect by recording an electron microscope image with high sensitivity on a two-dimensional image sensor such as a stimulable phosphor sheet, applying stimulating light or heat to the two-dimensional image sensor to cause the same to emit light, photoelectrically reading the emitted light to produce an image signal, and computing the focus defect from the image signal for the purpose of correcting the focus defect.
2. Description of the Prior Art
There are known electron microscopes for obtaining a magnified image of a specimen by deflecting a beam of electrons transmitted through the specimen with an electric or magnetic field. As is well known, the electron beam having passed through the specimen forms a diffraction pattern on the rear focal plane of the objective lens, and the diffracted beams interfere with each other again to produce the magnified image of the specimen. The magnified specimen image can be observed as a transmission image by projecting the image onto a screen with a projector lens. Alternatively, the rear focal plane of the objective lens may be projected for enabling the user to observe the magnified diffraction pattern of the image. Where an intermediate lens is positioned between the objective lens and the projector lens, the magnified transmission image or the diffraction pattern may be produced selectively as desired by adjusting the focal length of the intermediate lens.
The magnified image or diffraction pattern (hereinafter referred to collectively as a "transmitted electron-beam image) is generally observed by exposing a photographic film placed in the image formation plane of the projector lens to the transmitted electron-beam image, or by amplifying the transmitted electron-beam image with an image intensifier for projection. When the transmitted electron-beam image or electron microscope image is thus to be recorded on a recording medium or displayed on a display device, it is necessary to focus the image sharply. One general practice has been for the operator to observe an electron microscope image focused by projecting the image onto a fluorescent screen and, while observing this image, to manually turn a focus control knob so as to reduce any defocus present. According to another image focusing practice, the electron microscope image is projected onto an image intensifier and picked up by a television camera for display on a display unit such as a CRT, and the operator turns the focus control knob to eliminate the defocus while observing the displayed image.
However, the prior focusing techniques have proven unsatisfactory for various reasons. In order to display the electron microscope image on the fluorescent screen for enabling the operator to recognize a defocus, the specimen should be exposed to a relatively large amount of electron beams and is therefore liable to be damaged thereby. The electron microscope image displayed on the image intensifier to achieve a focused condition cannot be observed well since the displayed image has a low degree of sharpness and tends to be distorted. Another problem is that sole reliance on manual operation of the focus control knob while observing the displayed image to attain a well-focused condition is time-consuming and requires a certain level of skill on the part of the operator.
It is known to determine the size of a fringe from an electron microscope image recorded and developed on a photographic film. It is also known to subject such a developed electron microscope image optically to the Fourier transform for the measurement of a defocus. According to these known methods, however, it is necessary to employ a film developing process and an optical system, and the procedure is quite complicated.
The electron microscope, like the optical microscope, is affected by astigmatism. If it is desired to produce electron microscope images of high resolution, the astigmatism should strictly be corrected since it will otherwise impair the image quality. There have widely been used electron microscopes equipped with a stigmeter capable of correcting the astigmatism. The stigmeter comprises coils through which currents flow in respective X- and Y-axis directions, the currents being variable for astigmatism correction.
Therefore, the currents to be passed through the coils in the X- and Y-axis directions must be properly selected dependent on the astigmatism to be corrected. It has been customary to select the stigmeter currents while observing the granularity of the image of an amorphous material projected onto the fluorescent screen of the electron microscope, or based on the symmetry of a fringe produced when the image is brought out of focus. Inasmuch as these practices rely on the skill of the electron microscope operator, however, the astigmatism may not be properly corrected at times. Thus the electron microscope operator should be highly skilled.
To eliminate these shortcomings, it has been attempted to determine the extent of astigmatism quantitatively by employing an optically converted graphic pattern or figure generated by the Fourier transform of the image of the amorphous material. This process is based on the fact that the concentric ring pattern in the optically converted figure becomes elliptical in shape when astigmatism is present. More specifically, the extent of astigmatism .delta.z is given by: EQU .delta.z=(4n/.lambda.M.sup.2) (1/s.sup.2 -1/l.sup.2) (1)
where n is the degree of a ring having an intensity 0, counting from the lower angle of the concentric ring pattern, s and l are the lengths of the minor and major axes of the ring, .lambda. is the wavelength of the electron beam, and M is the magnification of the image. If the angle formed between the x and y axes and any desired direction serving as a reference for measuring .theta. is .pi./4, then the currents .delta.Ix, .delta.Iy to be passed through the stigmeter coils in the X- and Y-axis directions for eliminating the astigmatism .delta.z are expressed by: EQU .delta.Ix=C .delta.z.times.sin (.vertline..theta.-.alpha..vertline.-.pi./4) (2) EQU .delta.Iy=C .delta.z.times.sin (.vertline..theta.-.alpha.+.pi./4.vertline.-.pi./4) (3)
where C is a constant dependent on the relationship between the astigmatism and the stigmeter currents, .theta. the angle at which the astigmatism occurs, and .alpha. is an angle relating to the magnification M. Consequently, it is possible to determine precisely the stigmeter currents required for properly correcting the astigmatism when the extent of astigmatism .delta.z is quantitatively found. The above process of astigmatism correction is described in detail in, for example, "JOURNEY TO GENE OBSERVATION" edited by Hideo Yamagishi and published by The University of Tokyo, Publishing Society.
In order to produce the optically converted figure through the Fourier transform, it is necessary to expose a photographic film to the electron microscope image of an amorphous material, develop the image on the photograhic film, set the developed photographic image in a Fourier transform optical device, expose a photographic film to the optically converted figure, and finally develop the optically converted figure on the film. Such a procedure is quite complex. It is also necessary to measure the lengths s and l of the minor and major axes from the optically converted figure. As a consequence, the work of correcting astigmatism is laborious and time-consuming.